The present invention relates to a method for forming a resist pattern on a substrate by utilizing vapor phase graft polymerization.
In the conventional method for manufacturing ICs and LSIs, substrate surfaces are processed using a resist pattern of a polymer compound or the like. First, a resist film is formed on the surface of a substrate. The resist film is then irradiated with a high energy beam such as an ultraviolet ray, a far-ultraviolet ray, an X-ray beam or an electron beam. Then, an electrostatic latent image is formed in the resist film due to the chemical reaction. The image is developed to form the desired resist pattern.
With recent tendency toward farther micronization of a semiconductor element for an LSI, the resist material for forming such a resist pattern must have high sensitivity and resolution against an energy beam and must also have excellent etching resistance during processing of the substrate. This is because dry etching is more often used for processing a substrate since conventional wet chemical etching using an etching solution can only provide a low processing precision. However, requirements of sensitivity, resolution and etching resistance of the resist material cannot simultaneously be satisfied; a resist material which is excellent in all these properties has not yet been developed.
Micronization of LSI elements has also brought about another problem. When the wiring size for an element is decreased, the wiring resistance is increased and high-speed switching of the element may not be achieved. For this reason, the thickness of the wiring layer must be increased. This results in a significant step portion formed on the processed surface of the substrate. In order to process such a substrate surface, a resist film of a relatively great thickness must be used. However, the use of a relatively thick resist film generally results in a low resolution. This tendency is particularly notable when a negative-type resist is used which utilizes crosslinking. This is because swelling occurs at the crosslinked portion of the resist during wet developing such as spray developing or immersion in solvent. In the conventional method for forming an LSI element pattern by irradiation of a wafer with an electron beam or an ion beam, a negative-type resist may be more efficiently used. When the wet developing method is adopted, there arises another problem of low yield of element formation in addition to the problem of low resolution. The low yield is caused by formation of pinholes due to partial dissolution of the soluble resist component and by formation of defects due to dust or impurities in the developing solution.
In view of this situation, extensitive studies are currently made in an attempt to develop an effective material and method for forming a resist pattern by dry etching.
G. N. Taylor reported the following method in Journal of the Electrochemical Society, Vol. 127, pp. 2,665 (1980) and in Journal of Vacuum Science and Technology, Vol. 19, pp. 872, (1981). The resist material for forming a resist film consists of polydichloropropyl acrylate as an organic polymer compound and a monomer. The monomer is selected which reacts with the polymer upon irradiation of the polymer with a high-energy beam. After the resist layer is irradiated with the high-energy beam, it is heated in a vacuum to remove the monomer from the non-irradiated portion of the resist layer. When plasma etching is thereafter performed, the etching rate of the irradiated portion of the resist film is slower than that at the non-irradiated portion. This is because the monomer is contained in the irradiated portion of the resist film. Thus, the resist pattern may be formed. Many other literatures also report methods which combine dry etching and removal of a low-molecular compound by heating under a vacuum state similar to that used in the Taylor's method. However, in accordance with all these methods, the polymer material as a base of the resist film including both the irradiated and non-irradiated portions is the same. For this reason, a sufficient selective ratio beween the etching rates of these portions during dry etching may not be obtained, resulting in too far etching of the irradiated portions.
Harada et al has also proposed a method for forming a resist pattern on a substrate by dry etching in Japanese Patent Application No. 56-184495. According to this method, a resist film formed on a surface of a substrate is irradiated with a high-energy beam in a desired pattern to obtain an active point. Then, a radical addition polymerizable monomer is introduced to allow selective graft polymerization on the irradiated portion of the resist film. Using the thus obtained graft polymer pattern, the resist pattern is formed on the substrate by dry etching. According to this method of Harada et al, the resist film consists of a polymeric material which has low resistance to dry etching such as polymethyl methacrylate, and the graft polymer pattern consists of a polymeric material which has high resistance to dry etching such as polystyrene. Thus, this method utilizes the difference between the etching rates of these two polymeric materials. However, even with this method, in order to prepare a resist pattern with a good aspect ratio by forming a thick resist film, the graft polymer pattern must also be relatively thick. Then, the dose of the high-energy beam to be radiated onto the resist film must be increased and the time for graft polymerization must also be increased. Furthermore, since graft polymerization is anisotropic, deep etching also results in wide etching, resulting in a low resolution. Graft polymerization progresses in the resist film vertically as well as laterally, further degrading resolution.